Photovoltaic systems are arrangements of components designed to supply usable electric power for a variety of purposes, by converting solar radiation (sunlight) into usable direct current (DC) electricity. A PV array (also called a solar array) consists of multiple photovoltaic modules (solar panels). A photovoltaic system for residential, commercial, or industrial energy supply normally contains an array of photovoltaic (PV) modules, DC to alternating current (AC) power converters, a maximum power point tracker (MPPT), and optionally a battery system and a charger.
For any given set of operating conditions, PV cells have a single operating point where the values of the current and voltage of the PV cell result in a maximum power output. Maximum Power Point Tracking (MPPT) is a technique to extract the maximum available power out of the solar panels. Since the performance of solar cells depend on environmental conditions and have non-linear output efficiency, the MPPT system applies the proper loading conditions on the PV array such that maximum power is extracted at any operating point. Possible realization of MPPT can be obtained by apriori information of the PV system, or by sampling the output of the PV cells and adjusting the power converter's input characteristics to obtain maximum power for any given environmental conditions.
A load with resistance R=V/I equal to the reciprocal of this value draws the maximum power from the device. This is sometimes called the characteristic resistance of the PV cell. This is a dynamic quantity which changes depending on the level of illumination, as well as other factors such as temperature and the age of the PV cell. If the resistance is lower or higher than this value, the power drawn will be less than the maximum available, and thus the PV cell will not be used as efficiently as it could be. Maximum power point trackers utilize different types of control circuits or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a PV cell.
Serially connected PV arrays are widely used in such PV systems. However, full or partial shading of a serially connected PV array, severely impacts the power that can be extracted from the chain. Generally, two groups of power processing solutions have been proposed to resolve the shading problem.
“Per panel photovoltaic energy extraction with multilevel output DC-DC switched capacitor converters,” (J. J. Cooley, and S. B. Leeb, Twenty-Sixth Annual IEEE, APEC 2011, pp. 419-428) and “Improved Energy Capture in Series Chain Photovoltaics via Smart Distributed Power Electronics,” (Linares et al, Twenty-Fourth Annual IEEE APEC 2009, pp. 904, 910) propose power processing by assigning a dedicated converter/inverter per element in the PV array. However, this solution is costly, and requires that each of the dedicated converters will process the full power out of the PV element, which translates into increased losses.
“Generation control circuit for photovoltaic modules,” (Shimizu et al, IEEE Transactions on Power Electronics, vol. 16, no. 3, pp. 293-300, May 2001), “A returned energy architecture for improved photovoltaic systems efficiency,” (Nimni et al, Proceedings of 2010 IEEE ISCAS 2010, pp. 2191-2194) and “Centralized MPPT with chain current diverter for solving the series connection problem in photovoltaic power generation system,” Kadri et al, 2011 ICCEP, pp. 116-123, June 2011) disclose solutions that keep the series connection of the elements intact, and process mismatched currents due to the shaded unit(s) using parallel circuitry that channels the power difference to/from the main bus. By processing only power differences between the PV elements, conversion losses are minimized, while improving reliability.
A unique advantage of the differential power processing architecture (initially proposed in G. R. Walker, J. Xue, and P. Sernia, “PV String Per-Module Maximum Power Point Enabling Converters,” presented at the Australasian Universities Power Engineering Conference, 2003, and in G. R. Walker; J. C. Pierce, “PhotoVoltaic DC-DC Module Integrated Converter for Novel Cascaded and Bypass Grid Connection Topologies—Design and Optimisation,” 37th IEEE Power Electronics Specialists Conference, 2006 (PESC 2006), pp. 1, 7, 18-22 Jun. 2006) is that Maximum Power Point Tracking (MPPT) can be obtained locally, on a PV element level, by processing only the necessary amount of power needed to achieve Maximum Power Point (MPP). Several converter realizations have been proposed as candidates as Differential Power Processors (DPP), mainly derived from battery management applications.
A Switched Capacitor Converter (SCC), has a voltage equalizer with simple open-loop control, relying on the assumption that MPP voltage deviation is negligibly small due to change in irradiance level. This approach stands out in its simplicity, high self-efficiency and low cost. However, it lacks MPPT capability without introducing losses. A buck-boost topology has also been proposed, acting as an equalizer [R. Kadri, J. Gaubert, and G. Champenois, “Centralized MPPT with string current diverter for solving the series connection problem in photovoltaic power generation system,” 2011 International Conference on Clean Electrical Power, (ICCEP), pp. 116-123, 14-16 Jun. 2011], and further developed to obtain local MPPT by differential processing [P. S. Shenoy; K. A. Kim; B. B. Johnson; P. T. Krein, “Differential Power Processing for Increased Energy Production and Reliability of Photovoltaic Systems,” IEEE Transactions on Power Electronics, vol. 28, no. 6, pp. 2968, 2979, June 2013], in order to keep all PV units in MPP. However, compared to SCC technology of the same power level, it is bulkier in volume due to the large magnetic element required.
Converter topologies such as switched-capacitor type (described for example in “A resonant switched-capacitor converter for voltage balancing of series-connected capacitors,” by Sano et al., International Conference on PEDS 2009, pp. 683, 688) or a buck-boost type (described for example in “A Review of Cell Equalization Methods for Lithium Ion and Lithium Polymer Battery Systems”, Moore et al., SAE 2001 World Congress, No. 2001-01-0959, March 2001), were originally proposed for battery equalization and realized for PV applications have many merits. However, their efficiency range is limited and SCC configuration lacks MPPT capability. In a buck-boost configuration, the efficiency range is somewhat limited around the nominal power level, and there is a trade-off between the size and performance of the converter.
All the conventional methods described above failed to propose a differential power processing converter for PV systems, which combines small dimensions and high conversion efficiency over a wide range.
It is therefore an object of the present invention to provide a new DPP topology that overcomes the limitations of conventional DPPs.
It is another object of the present invention to provide a new DPP topology that combines the benefits of reduced size SCC and current sourcing properties with high efficiency over a wide range.
It is a further object of the present invention to provide a new DPP topology that is fully capable of performing local MPPT with SCC technology.
It is a further object of the present invention to provide a new DPP topology in which only the mismatch power between PV elements is processed while converging each of the elements into its corresponding MPP.
Other objects and advantages of the invention will become apparent as the description proceeds.